This invention relates to a television viewing device and, more particularly, to such a device which receives television signals, reproduces images from those signals, and optically projects those images onto a compact viewing screen.
Among the many problems of projecting a real image of a small television tube face over a wide range of distances, focusing it always on a wide screen, is that the apparent brightness of the image drops rapidly as the screen is moved away. Another important consideration is that the cathode ray beam "spot size" is measured as a function of the picture height. As the size of the picture tube gets smaller, the diameter of the spot must go down proportionally, if we demand that the vertical resolution in the picture does not degrade. However, if we want the same screen brightness, we need the same number of electrons hitting per unit area on the screen in a unit of time. Thus, when the spot size shrinks in area, the required beam current decreases as the picture height, squared. Conversely, if the beam current remains the same, the picture becomes brighter as the picture size shrinks in area. This analysis leaves out a number of parameters that influence screen brightness, but in general, it is easier to produce a high resolution, bright picture in a small size than it is in large sizes.
Furthermore, a miniaturized screen, with an appropriate optical display, lends itself to a closed circuit, "color wheel" type system in which a black and white CRT field sequentially displays images which are sequentially viewed by an observer through a revolving color filter wheel whose revolutions are synchronized with the field scanning rate of the CRT display.
A rotating color wheel display has two separate and important advantages over the standard shadow mask technology. The first is that no adjustments are needed to make all three "subfields" appear in exact registration over the whole area of the display. Shadow masks require the use of electron optical systems whose optical axes are inclined to each other. This feature makes it necessary to employ circuits to eliminate "keystoning," the distortion of a rectangular raster into a trapezoidal shape. This feature also insures that electron beams corresponding to the same spot in the three primary colors traverse different paths of the deflecting fields which causes a different kind of raster misregistration that must also be corrected to give the best possible image. The second advantage is that there is no structure to the image due to the coarse mask structure. In raster scanned displays, the raster line structure causes a much smaller degradation of the image, and it can be rendered invisible by increasing the number of scanning lines to match the resolution of the human eye. In "vector scanned" displays, the rotating color wheel display will yield the most perfect color picture ever seen, with neither "color fringing" nor image degradation due to mask structure or scanning live structure.
In the earliest rotating color wheel systems, the face of the cathode ray tube was viewed directly, so it was necessary to employ a color wheel larger in radius than the actual screen size. Such a structure would not be acceptable to present day users. In the present invention, with its use of specially designed projection optics, the rotating wheel can be smaller in diameter than the height of the final viewing screen, so it does not require a "bulge" in the cabinet. And the wheel can be placed at many different points along the optical axis, at the choice of the designer.